1. Field of the Invention
This invention relates to the field of fiber optics and, more particularly, to the use of fiber optics in a fire sensing system.
2. Description of the Prior Art
The technology of fiber optics finds application in a great many fields. Since 1970, when researchers at Corning Glass Works announced the first low loss optical fiber (less than 20 dB/km) in long lengths (hundreds of meters), the fiber optics industry has been experiencing an explosive growth. Communications applications have been dominant and are therefore primarily responsible for sparking the technological development.
The principle upon which fiber optics depend for their effectiveness is that of total internal reflection. An optical fiber consists of a cylindrical core of material (usually glass or plastic) clad with a material (either glass or plastic) of lower refractive index, thus preventing light loss through the exterior surface for incident light within the fiber acceptance cone.
A second principal feature of optical fibers contributing to their broad application in various fields of use is the extreme thinness of the fiber which enables it to be very flexible. Optical fibers typically are fabricated to diameters as small as 5 microns and ranging upward to 500 microns or more. These fibers are then typically assembled in bundles or cables, sometimes referred to as "light guides", which still exhibit substantial flexibility and can be used for various purposes.
Many technical applications of fiber optics use either "incoherent" or "coherent" bundles of fibers. In an incoherent light guide, there is no relationship between the arrangement of the individual fibers at the opposite ends of the bundle. Such a light guide can be made extremely flexible and provides a source of illumination to inaccessible places. When the fibers in a bundle are arranged so that they have the same relative position at each end of the bundle, the light guide is known as coherent. In this case, optical images can be transferred from one end to the other.
Thus, optical fiber transmission systems find a wide variety of uses such as, for example, in the interconnection of telephones, computers and various other data transmission systems (communications); in the fields of instrumentation, telemetry and detection systems; and in the medical field (bronchoscopes, endoscopes, etc.), to name but a few. For example, in the field of medical instrumentation, an incoherent light guide offers the best means of safely illuminating a point inside the body, since it provides light without heat. A coherent light guide can be used in conjunction therewith for observation or photography.